The present invention relates to a liquid crystal display device and, more particularly, to an improvement in a field effect liquid crystal display device for time-multiplexed driving.
A conventional so-called twisted nematic liquid crystal display device has a 90.degree. twisted helical structure of a nematic liquid crystal having positive dielectric anisotropy and sealed between two substrates having transparent electrodes arranged thereon in desired display patterns. Polarizing plates are arranged on outer surfaces of the substrates such that polarizing axes (or absorption axes) thereof become perpendicular or parallel to the major axes of the liquid crystal molecules adjacent to the substrates.
In order to twist the liquid crystal molecules between the two substrates through 90.degree., for example, a so-called rubbing method is utilized to rub a surface of a substrate which contacts the liquid crystal molecules by a cloth along one direction. In this case, the major axes of the liquid crystal molecules adjacent to the surface become parallel to this one direction (i.e., a rubbing direction). Two rubbed surfaces are spaced apart so as to oppose each other while their rubbing directions are twisted through 90.degree.. These rubbed substrates are then sealed with a sealing agent, and a nematic liquid crystal having positive dielectric anisotropy is filled in a space formed between the substrates. Therefore, the major axes of the liquid crystal molecules are twisted through 90.degree. between the substrates. The polarizing plates disposed on the resultant liquid crystal cell have polarizing or absorption axes substantially parallel to the liquid crystal modelcules adjacent thereto, respectively. In a conventional reflective type liquid crystral display device which is most frequently used, a reflector is disposed on the outer surface of the lower polarizing plate. Light incident from the upper surface of the device is linearly polarized by the polarizing plate or polarizer. In a portion of a liquid crystal layer which is not applied with a voltage, the plane of polarization of the linearly polarized light is rotated through 90.degree. along the helical structure and is transmitted through the lower polarizing plate or analyzer. The light is then reflected by the reflector and returns to the upper surface of the device. However, in a porton of the liquid crystal layer which is applied with a voltage, when the helical structure is destroyed, the plane of polarization of the linearly polarized light will not be rotated. Therefore, the linearly polarized light transmitted through the upper polarizing plate is blocked by the lower polarizing plate and will not reach the reflector. In this manner, electrical signals can be converted into optical images in accordance with the presence or absence of an electrical potential applied across the liquid crystal layer.
Performance parameters for a quantification of time-multiplexed driving in a subsequent description will be briefly described below.
FIG. 1 is a graph showing typical voltage-luminance characteristics of a conventional reflective type liquid crystal display device. The graph shows the relative luminance of reflected light as a function of the applied voltage. An initial value of luminance is 100%, and the final value (a value at a sufficiently high applied voltage) is 0%. In general, a threshold voltage Vth is given at a 90% relative luminance, and a saturation voltage Vsat is given at a 10% relative luminance so as to determine the liquid crystal characteristics. However, in practice, a pixel is sufficiently bright when the relative luminance is more than 90%, so that the pixel is considered to be in an OFF state. When the relative luminance is less than 50%, the pixel is sufficiently dark, and hence the pixel is considered to be in an ON state. Voltages corresponding to 90% and 50% of relative luminances are given as the threshold voltage Vth and the saturation voltage Vsat, respectively, hereinafter. In other words, the threshold voltage Vth is given as a maximum allowable voltage corresponding to the OFF state, and the saturation voltage Vsat is given as a minimum allowable voltage corresponding to the ON state.
The electrooptical characteristics of the liquid crystal display device change in accordance with a viewing angle. These characteristics limit a viewing angle range within which a good display quality is obtained.
A viewing angle .phi. will be described with reference to FIG. 2. Referring to FIG. 2, a rubbing direction of an upper substrate 11 of a liquid crystal display device 1 is represented by reference numeral 2, a rubbing direction of a lower substrate 12 is represented by reference numeral 3, and a twist angle between liquid crystal molecules adjacent to the upper substrate and liquid crystal molecules adjacent to the lower substrate is represented by reference numeral 4. X- and Y-axes are plotted along the surface of the liquid crystal display device 1. The X-axis defines a direction for bisecting the twist angle 4 of the liquid crystal molecules. A Z-axis defines a normal to the X-Y plane. An angle between a viewig direction 5 and the Z-axis is defined as the viewing angle .phi.. In this case, by way of simplicity, the viewing direction 5 is plotted in the X-Z plane. The viewing angle .phi. in FIG. 2 is regarded to be positive. Since contrast becomes high when viewed from a direction within the X-Z plane, this direction is called the viewing direction 5.
Conventional commercially available liquid crystal display devices have viewing angles falling within a range of 10.degree. to 40.degree.. Therefore, referring to FIG. 1, when a voltage corresponding to the 90% luminance at the viewing angle .phi. of 10.degree. is represented as a threshold voltage Vth1, the voltage corresponding to the 50% luminance at the same viewig angle is represented as a saturation voltage Vsat1, and a voltage corresponding to the 90% luminance at a viewing angle of 40.degree. is represented by a threshold voltage Vth2, the sharpness of the luminance-voltage characteristic, .gamma., the viewing-angle D dependence, .DELTA..phi., and the time-multiplexability, m, are defined as follows:
.gamma.=Vsat1/Vth1 PA1 .DELTA..phi.=Vth2/Vth1 EQU m=Vth2/Vsat1
Assuming luminance-voltage characteristic curves are ideal, the two curves at viewing angles .phi. of 10.degree. and 40.degree. do not differ, the curves are steep enough for a threshold voltage and a saturation voltage to have the same value.
The time-multiplexed driving of the conventional liquid crystal display device depends on .DELTA.n.multidot.d where .DELTA.n is the refractive index anisotropy, i.e., optical anisotropy of the liquid crystal and d is the distance between the upper and lower substrates. When .DELTA.n.multidot.d is large (e.g., more than 0.8 .mu.m), the sharpness of the luminance-voltage characteristic .gamma. becomes good (small), and the viewing-angle dependence .DELTA..phi. is poor (small). However, when .DELTA.n.multidot.d is small (e.g., less than 0.8 .mu.m), the sharpness of the luminance-voltage characteristic .gamma. becomes poor (large) and the viewing-angle dependence .DELTA..phi. becomes good (large). However, the time-multiplexability m (=.DELTA..phi..gamma.) becomes good (large) when .DELTA.n.multidot.d is decreased. A typical example is summarized in Table 1.
TABLE 1 ______________________________________ .DELTA.n .multidot. d Characteristics 0.5 .mu.m 1.0 .mu.m ______________________________________ .gamma. 1.150 1.084 .DELTA..phi. 0.965 0.877 m 0.839 0.808 ______________________________________
Time-multiplexed driving will be briefly described with reference to a dot matrix display. As shown in FIG. 3, Y stripe electrodes (signal electrodes) and X stripe electrodes (scanning electrodes) 14 are formed on the lower and upper substrates 12 and 11, respectively. The liquid crystal portions at intersections of the X and Y electrodes 14 and 13 are chosen to be in an ON state or an OFF state so as to display characters or the like. Referring to FIG. 3, n scanning electrodes X1, X2, . . . , Xn are repeatedly scanned in the order named in a time-multiplexed manner. When a given scanning electrode (e.g., X3 in FIG. 3) is selected, a selection or nonselection display signal is simultaneously applied to all pixels P31, P32, . . . and P3m on the given scanning electrode through the signal electrode 13 constituted by electrode Y1, Y2, . . . and Ym in accordance with a display signal. In other words, the on/off operation of the pixels at the intersections of the scanning electrodes and the signal electrodes is determined by a combination of voltage pulses applied to the scanning and signal electrodes. In this case, the number of scanning electrodes X corresponds to the number of time-multiplexing.
The conventional liquid crystal display device has poor time-multiplexed drive characteristics as shown in Table 1, and these characteristics would permit time-multiplexing of only a maximum of 32 or 64. However, demand has arisen to improve the image quality of the liquid crystal display device and increase the number of data to be displayed. Any conventional liquid crystal display device cannot satisfy these needs.